Ion generation devices are very widely used in the industrial field for carrying out surface treatments (ion etching, cleaning, deposition of material, ion implantation, etc.) and chemical and physical analysis (for example, the determination of the type and orientation of crystals on the surface of a solid). In the space field, such devices are used as ion engines and, on earth, for the generation of simulated ionospheric plasma. An ion generation device is represented schematically in FIG. 1. It consists of an ionization chamber 1 and an extraction system 2. A substance in the form of a gas or vapor is introduced into the ionization chamber, and the (positive) ions of the desired chemical nature are obtained from the substance by various techniques which are known in themselves. These ions are then extracted from the ionization chamber, focused, and accelerated toward the objective of the extraction system 2.
Other parts may be present in the device, but will not be mentioned here since they are not relevant to the description of the invention which is the subject of the present patent application. However, it is appropriate to describe the extraction system 2 in greater detail. Said extraction system has the object of producing, at the outlet of the ionization chamber, a configuration of the electrical field (and in some cases of the magnetic field also) such as to accelerate the ions to the desired velocity and to focus the ion beam, according to the application. In addition, the extraction system must limit the flow of neutral atoms in order to improve the ionization efficiency, that is to optimize the outgoing ion flow with respect to the incoming neutral atom flow.
The configuration of the extraction system may or may not comprise one or more grids. The solution with a plurality of grids is most generally used and is necessary where there is a requirement for a high ionization efficiency and/or a high quality of the beam produced in terms of uniformity of energy of the ions and accuracy of focusing. Two or three grids are usually present, although devices with one and with four or more grids have also been produced. Of these grids, a certain number are maintained at strongly negative electrical potentials and have the purpose of accelerating the ions to form the beam and to prevent the exit of negative charges from the ionization chamber.
One of the most serious problems associated with a grid extraction system is that of sputtering, in other words the removal of material from the grids, and consequently their erosion, as a result of the bombardment of high-energy ions. This phenomenon becomes more marked as the energy of the incident ions increases, in other words as the electrical potential of the grids becomes more negative. The erosion of the grids necessitates, in the case of ion generators for industrial use, costly maintenance operations (inspection and periodic replacement of the grids and of the insulators on which the removed material is deposited) and also forms a source of impurities in the ion beam. In the case of ion engines for space applications, the phenomenon is clearly one of the most serious factors limiting their service life.
In the construction of grids, therefore, it is necessary to use material with a low sputtering coefficient, in other words those which suffer least from the erosion effect due to the incidence of high-energy ions.
At present, grids are constructed from sheets of materials such as steel, molybdenum, carbon or other material, of sufficient thickness to ensure the operability of the grid in the presence of erosion for the desired operating period. These sheets are cut to suitable sizes and must then be perforated according to a pattern which, in the larger models, requires thousands or tens of thousands of holes. The perforation is carried out by various techniques, but in all cases requires long periods and is generally very expensive, but above all produces relatively low values of transparency. Furthermore, the optical characteristics, in other words the ion beam focusing characteristics, of grids produced by this technique are not optimal.